With polls suggesting a hung parliament after Britain's 6 May election, the leader of the Liberal Democrats warned he would not prop up a weakened Labour Prime Minister.
Nick Clegg's party, which has surged from third to second place in opinion polls after he performed strongly in TV debates, is unlikely to win enough votes to form a government after the closely-fought battle.
It could, though, hold the balance of power in a hung parliament -- where no party wins an overall majority, not seen since 1974 - and team up with Labour or the Conservatives to govern.
The Liberal Democrats have never said which party they would team up with in the event of a hung parliament although Mr Clegg has said the party with the strongest ‘mandate’ should be allowed to try to govern.
But in a strong message to Mr Brown today, Mr Clegg warned Labour that he would not support them if they dropped to third in the general election.
‘It is just preposterous the idea that if a party comes third in the number of votes, it still has somehow the right to carry on squatting in Number Ten (Downing Street),’ he told the BBC.
‘I think a party which has come third - and so millions of people have decided to abandon them - has lost the election spectacularly (and) cannot then lay claim to providing the prime minister of this country.’
Mr Clegg also told the Sunday Times that Labour was 'increasingly irrelevant’.
He spoke as opinion polls suggested the Conservatives, led by David Cameron, had extended their lead, though not by enough to win an outright majority, which would mean a hung parliament.
One of them, a YouGov survey in the Sunday Times, put Tories on 35%, a rise of two points, with the Liberal Democrats down one on 28% and Labour down three on 27%.
Mr Cameron, meanwhile, said he would not support the Liberal Democrats' top demand - reforming the voting system - sending a signal that the two parties would find it very hard to work together, although he did not rule that out.
‘I want us to keep the current system that enables you to throw a government out of office. That is my view', he told the Observer newspaper. ‘We think this is an important issue.’
Business Secretary Peter Mandelson, seen as Mr Brown's de facto deputy, warned supporters of centre-left Labour who were thinking of switching support to the centrist Liberal Democrats that doing so could let in Mr Cameron.
‘You might start flirting with Nick Clegg, but that way you will end up marrying David Cameron,’ he told the Sunday Mirror.
Disillusioned Labour voters in Britain sometimes switch to the Liberal Democrats in protest as both are seen as progressive parties.
This happened most recently in the 2005 general election, when the Liberal Democrats saw a surge of support due to their opposition to the Iraq war, which the Labour government under Tony Blair led Britain into.
Mr Brown said yesterday he was ‘upping the tempo’ in the election campaign with less than two weeks to go until polling day.
